1. Technical Field
This disclosure relates to electronic control circuits, and more particularly to a method of controlling a power factor correction converter and a related closed-loop control system of a power factor correction converter.
2. Description of the Related Art
Basically, a common example as depicted in FIG. 1, a transition mode (TM) power factor correction converter (PFC) may comprise a boost inductor L, a switch M, a diode D, an output tank capacitor Cout. In the contemplated example, an auxiliary winding Laux is magnetically coupled with the inductor as one exemplary way for sensing a condition of zero current (or zero crossing) in the boost inductor.
However, other ways, equally familiar to the skilled reader, may be chosen, for example a sense resistor in a recirculation current path of the boost inductor through the ground node may be used in lieu of an auxiliary winding.
Control circuit means, commonly based on a microcontroller, may be used for controlling the generation and delivery of a drive signal to the GD node, commonly a square wave, for turning on the switch M when a zero current condition through the inductor is detected (in the considered example by monitoring the voltage on the ZCD sense node), and for turning off the switch after an on-time interval (Ton) set by the controller has elapsed. One may monitor on the sense node CS the current that charges the boost inductor L during the on-time pulse applied to the gate of the switch M.
The basic circuit of FIG. 1, also shows a common condition of direct AC line feed of the switching PFC power converter through a full bridge rectifier and filter capacitor C1.
The PFC generates a fixed DC output voltage Vout. The average current absorbed from the input mains typically has a rectified sinusoidal shape of a desired amplitude in phase with the rectified input voltage Vin.
FIG. 2 shows the basic circuit (PFC Plant) of the PFC power converter of FIG. 1 and first circuit means CONTROL LOOP for controlling it.
FIG. 3 shows a typical discontinuous current waveform for a cycle of a rectified AC sinusoidal feed voltage input to the PFC power converter operating in a transition mode (TM). The output DC power and input current waveform are controlled by regulating the duration of the on-pulse of the gate drive signal that controls the switch. The on-time interval of the switch is commonly set by the controller in function of the output power. Therefore, if the load remains constant the on-time remains constant too. Because the current at the beginning of the cycle is constant, the peak current is Ton*Vin/L, that is, it is proportional to the input voltage (which is in fact the target behavior of a PFC).
The duration of the off-time depends upon the current flowing through the boost inductor L and is sufficient to let this current go to zero. The transition mode peculiarity of zeroing the current IL in the inductor at every switching cycle, has the drawback of producing a current ripple of amplitude that is about twice the average value of the power transfer current (i.e., the average current).
In order to improve efficiency, PFC are controlled with the following techniques:                Off-time modulation to linearly decrease the switching frequency under light load conditions (for example, as in the device SG6846 of System General). This modulation allows to improve efficiency reducing switching frequency. Anyway in order to guarantee electromagnetic interference (EMI) specifications, tone can put an higher limit to the minimum reachable switching frequency. Efficiency gain is consequently limited;        Skip mode technique that allows to skip cycles near zero crossings of the output voltage where the current is very low. This is obtained using a comparator with a fixed threshold (for example as in the device NCP1611 of ON-semi). This technique with fixed threshold increases current distortion and worsens the power factor;        Burst functioning mode. This technique is largely implemented under very light load condition because it appears more efficient than the above techniques when the load is very small.        
It is very hard to meet efficiency requisites that are nowadays imposed by international standards, especially at very light loads. It is still needed a method of controlling a power factor correction converter implemented in a relative control system that allows to maximize efficiency whilst meeting the most recent norms and International standards that impose a limit on harmonic content, PF (power factor) and EMI (electromagnetic interference).